Since PTFE has properties such as high chemical resistance and low conductivity, a high melting point and excellent heat resistance, it is used for a variety of purposes mainly in chemical and electric fields. It is widely used for machinery purposes such as non-lubricated sliding components taking advantage of the property of a small friction coefficient or surface tension.
One type of PTFE product is a sheet-like product (PTFE sheet), which finds many applications depending on its form, including shape and thickness. For example, a strip-like PTFE sheet with a thickness of about several ten to several hundred micrometers suitably can be used as a seal tape to create a seal at the joint of pipes such as water pipes and gas pipes. The seal tape needs to be flexible enough to be forced into gaps at the joint, where it is required to serve as both a lubricant and a seal. The PTFE seal tape, with such properties and functions, widely has been used under the common name “raw tape” or “packing material.”
On the other hand, PTFE does not dissolve in most solvents except for specific solvents, and has the melting viscosity as high as in a range from 1010 Pa·s to 1011 Pa·s (from 1011 P to 1012 P) at a temperature of 380° C. Thus, various forming methods (such as extrusion and injection molding) used for forming general thermoplastic resin are not applied easily for manufacture of a PTFE product. These forming methods work with resin having melting viscosity upon formation generally in a range from 102 Pa·s to 103 Pa·s.
In one traditional method of manufacturing a PTFE sheet, sinter forming and cutting are used in combination. In this method, powdery PTFE particles (molding powder), used as a starting material, first are preformed at room temperature (forming aids may be added as needed at this point), and the resulting preform is heated at a temperature equal to or more than the melting point of PTFE (about 327° C.) to sinter (fire) the whole and obtain a cylindrical PTFE block (sinter forming). This is followed by cutting the outer periphery of the formed block to obtain a PTFE sheet (cutting method). Although a sheet with a relatively large thickness (for example, not less than 25 μm) may be obtained according to this method, the block size has to be enlarged for manufacturing sheets efficiently. In that case, preforming and firing take a long time (approximately from two to five days, depending on the block size) to inhibit the development of defects such as cracks due to heat distortion. Moreover, since the method is basically a batch process, continuous manufacture of a PTFE sheet from the starting material is difficult.
Another method of manufacturing a PTFE sheet is a paste extrusion method (fine powder method), which commonly is used to manufacture a PTFE seal tape. In this method, a forming aid is added to a fine powder to form a rod-like preform, which is then extruded with an extruder. The extruded paste then is spread into a sheet of a predetermined thickness using rollers, before the forming aid vaporizes. A PTFE seal tape is obtained by removing the forming aid from the sheet. A drawback of the paste extrusion method, however, is that it poses environmental concerns over the organic solvent, such as oil, used as the forming aid. Further, the method is not productive because it is a batch process requiring separate processes for the step of forming a preform and the step of extruding the preform.
JP5(1993)-301267A (Document 1) discloses a paste extrusion method using water as a forming aid. As described in Document 1, the method forms a paste suited for extrusion by removing water from a slurry containing an aggregate of PTFE and water, and uses this paste for extrusion. In order to reduce the content of water in the slurry, the slurry is passed slowly through a pipe provided with a slit or a gap of the shape and width that allow for passage of water but block the PTFE aggregate. Since there is no commercially available device that can realize such a procedure, the cost of equipment would be high if the method of Document 1 were to be performed. Further, a step of aggregating the PTFE will be needed when the slurry containing the PTFE aggregate and water is to be prepared from a dispersion of PTFE. Such a step is possible by, for example, adding a coagulant, such as an acid, to the dispersion heated to a predetermined temperature, and agitating the dispersion with an agitator. This step needs to be performed separately from the step that removes water from the slurry, which is a big obstacle to improving productivity.
Casting is another method of manufacturing a PTFE sheet. In casting, a dispersion of PTFE particles (a PTFE dispersion), which is a starting material, is applied on a support such as a metal plate and then dried and fired, followed by separating from the support for obtaining a PTFE sheet. According to this method, compared to the foregoing methods, a PTFE sheet is obtained that is thinner and less distorted. The thickness of a sheet obtained by one procedure of application, drying and firing, however, is considered to have an upper limit of about 20 μm for inhibiting minute defects called mud cracks, and a sheet with a thickness above 20 μm has to be obtained by repeating the application and firing of the dispersion a plurality of times.
The cutting method, paste extrusion method, casting and other methods of manufacturing a PTFE product are described in, for example, “Handbook for Fluororesin” (Takaomi Satokawa, ed., published by Nikkan Kogyo Shimbun, Ltd. in 1990 (description for the paste extrusion method on pages 122 to 124, for the cutting method on pages 141 to 142, for casting on page 130)).
Though not intended for the manufacture of a PTFE product, there are methods that apply a mechanical force to the dispersion of PTFE particles or the dispersion of fluorinated thermoplastic resin to form secondary particles (for example, PTFE fine powder) having a larger particle size than the original particles, as disclosed in JP2002-201217A (Document 2), JP6(1994)-192321A (document 3), and JP2003-522230T (Document 4). JP47(1972)-12332A (Document 5) discloses a method in which PTFE secondary particles having a larger particle size than the original particles are formed by applying a mechanical force after wetting the PTFE particles with an aqueous solution containing a surfactant.